Fix to delete saved ipynb from the list of files to be render.

A little hack to avoid problems with the saved ipynb files inside a hidden folder called ipynb_checkpoints (yes IPython write *states* of the notebook in a hidden folder to *save* the the current notebook).
Nikola tries to render, but this folder is garbage at the time of render the posts. So, I *clean* any ocurrence of files living inside .ipynb_checkpoints folders, from the `full_list` containing the sources files of the posts.

Cheers

Damián.